Chemical tailoring of proteins is a powerful approach to investigate structure function relationships and the role of post-translational modifications.1-6 Protein semi-synthesis4,7 and total synthesis8 are commonly used to introduce novel functionalities into proteins. Both approaches rely on native chemical ligation (NCL)—a chemoselective amide forming reaction between an αthioester and an N-terminal cysteine moiety.9 When protein αthioesters are generated from engineered inteins and then modified with NCL the process is referred to as expressed protein ligation.10-12 Inteins are protein self-splicing elements that can be engineered to generate protein αthioesters after self-cleavage in the presence of small molecule thiol.10-12 Despite a number of new methods for the chemical synthesis of peptide αthioesters, intein mediated synthesis of protein αthioesters is the only route to generate this important functionality needed for semi-synthesis.